1. Field of the Invention
The present invention relates to an image scanning apparatus for scanning an image on an original, a control program for controlling the same and a storage medium storing the control program.
2. Description of the Related Art
In an image scanning apparatus for acquiring a digital image by scanning an image on an original, a CCD photodetector has been heretofore used as a light receiving section of a scanning optical system for scanning the original. Herein, the original is a film, a printing original, and the like, and the image scanning apparatus is a so-called scanner.
FIG. 12(a) is a diagram showing a photoelectric-conversion characteristic of the CCD photodetector, that is, a relation between an exposure and an intensity.
The intensity R indicated by an output signal of the CCD photodetector becomes higher as the exposure I of the CCD photodetector increases. However, when the exposure I exceeds a quantization exposure limit Ia, the intensity R saturates at a constant value Ra.
Furthermore, when the exposure I exceeds an exposure Iaxe2x80x2 slightly higher than the quantization exposure limit Ia, an xe2x80x9coverflowxe2x80x9d occurs.
Herein, the xe2x80x9coverflowxe2x80x9d signifies a phenomenon that charges are flown over the surroundings from a pixel in the CCD photodetector in which signal charges are to be held and an analog shift register in the CCD photodetector.
When this xe2x80x9coverflowxe2x80x9d occurs at any one of the plurality of pixels or at any one of the analog shift registers, a phenomenon called smear or blooming (hereinafter referred to as xe2x80x9coverflowxe2x80x9d phenomenon) occurs in an image finally obtained.
On the contrary, when the exposure I is insufficient, a dynamic range of the CCD photodetector is not effectively utilized, which deteriorates the SN ratio of an image signal and an overall quality of a finally obtained image.
Accordingly, in order to set an exposure condition that is proper (hereinafter referred to as a proper exposure condition), the conventional image scanning apparatus examines the intensity histogram of an image to be scanned before scanning it.
Herein, a pre-scan must be executed to examine the intensity histogram.
The pre-scan is a scanning performed under a predetermined exposure condition (specifically, an exposure condition in which an intensity does not saturate at scanning any image, that is, an exposure does not exceed a quantization exposure limit Ia in scanning any image.
Incidentally, the exposure condition is determined depending on a combination of a drive condition of an illumination optical system and a drive condition of a CCD photodetector, that is, a combination of a scanning light amount and a charge storage time. Accordingly, a predetermined exposure condition is set according to a predetermined scanning light amount and a predetermined charge storage time.
FIG. 12(b) is a drawing for explaining an original. Descriptions are made in the following for the case where the original to be scanned is a negative film.
In each film, a position of an image frame (an area in which an image is stored) 12a on the film varies depending on sorts of cameras used for photographing.
Accordingly, a scanning area 12E that is determined in advance in the image scanning apparatus has a range slightly wider than the image frame 12a. 
FIG. 12(c) shows an intensity histogram that is acquired by a pre-scan.
The intensity histogram illustrates the intensity of the scanning area 12E ranges from 0 to Rmax. This range corresponds to a part of the dynamic range of the CCD photodetector, which ranges from 0 to Ra.
The image scanning apparatus refers to the intensity Rmax of the brightest point in the scanning area 12E (hereinafter to be referred to as xe2x80x9can absolute brightness pointxe2x80x9d).
The image scanning apparatus sets the exposure condition so that the dynamic range of the CCD photodetector is effectively utilized in the scan, and in addition the exposure by this absolute brightness point does not exceed the an exposure limit.
To be specific, the image scanning apparatus sets the exposure condition to have the exposure in the scan Ra/Rmax times as much as the exposure in the pre-scan.
The exposure condition newly set described above is a proper exposure condition in the conventional image scanning apparatus.
FIG. 12(d) is an intensity histogram acquired by the scan.
This intensity histogram shows clearly that an intensity range of the scanning area 12E after the scan is expanded to the whole area of the dynamic range of the CCD photodetector, which ranges from 0 to Ra.
As a result, the S/N ratio of an image signal is enhanced to the maximum within a range where the overflow phenomenon does not occur.
Incidentally, as described above the scanning area 12E is set slightly wider than the image frame 12a. 
Accordingly, the scanning area 12E includes not only the image frame 12a that is an effective picture but also a base 12b that is a non-effective picture.
Herein, the effective picture refers to a picture to be expressed in tones, and the non-effective picture refers to a picture which has a very high intensity compared to the effective picture and needs not to be expressed in tones.
Incidentally, the intensity histogram shown in FIG. 12(c) and FIG. 13(a) to be described later has two peaks.
Of these two peaks, the gentle peak on the low intensity side corresponds to intensity information of the image frame 12a, and the steep peak on the high intensity side corresponds to intensity information of the base 12b. 
Accordingly, the intensity Rmax at the absolute brightness point is equal to an intensity of the base 12b. 
Herein, if an object to be scanned is a comparatively bright image, an intensity in the intensity range of the image frame 12a is greatly lower than the intensity Rmax of the base 12b, as shown in FIG. 13(a).
However, the foregoing proper exposure condition is determined in accordance with the intensity Rmax at the absolute brightness point (the intensity at the base 12b), which may be a cause of the overflow. The proper exposure condition should not be determined in accordance with the intensity at the brightest point (hereinafter referred to as xe2x80x9cthe brightest point of picturexe2x80x9d) of the effective picture (image frame 12a).
Therefore, when the intensity range of the image frame 12a is significantly lower than the intensity Rmax of the base 12b, the intensity range allocated to the image frame 12a after the scan is still narrow as shown in FIG. 13(b), even when the foregoing proper exposure condition is set.
In this case, it has to be permitted a situation that the S/N ratio of the image signal remains small.
Furthermore, such a problem may occur in positive films in addition to negative films.
Such a problem arises, especially, when images such as a starry sky and metal goods are objects of scanning.
This is because these images contain a luminescent point that is a non-effective picture.
The same problem may occur regardless of types of the images to be scanned in a flatbed scanner (an image scanning apparatus where a film is placed on a transparent original stage larger than the film).
This is because, in the flatbed scanner, a full transmission area where the film is not placed is scanned together with an area where the film is placed.
To be more specific, the full transmission area becomes a non-effective picture having a significantly high intensity compared to an effective picture.
Note that, users of the flatbed scanner visualize images obtained by the pre-scanning, and distinguish the non-effective picture such as the foregoing base, luminescent point, and full transmission area from the effective picture. The users can cover the non-effective picture with a prepared mask.
However, this is a complicated processing for the users. Besides, it is very difficult to cover minute luminescent points with the mask.
An object of the present invention is to provide an image scanning apparatus capable of obtaining an image signal in high quality even when a non-effective picture exists in a scanning area, a control program for controlling the same, and a storage medium for storing the control program.
To achieve the objects, the image scanning apparatus in the present invention comprises a radiation unit for radiating a scanning light onto a scanning area where an original is arranged; a photodetector for detecting an optical image formed by the scanning light which is reflected in or transmits through the scanning area; a test drive unit for driving the radiation unit and the photodetector for which a predetermined drive condition is set; and an adjusting unit for adjusting the drive condition for the radiation unit and the photodetector in accordance with an output signal outputted from the photodetector by the driving, wherein the photodetector is a photodetector having an overflow drain mechanism, and the adjusting unit adjusts an exposure of the photodetector at a brightest point of an effective picture of the optical image so as to approach the quantization exposure limit of the photodetector. Accordingly, even if a non-effective picture exists in a scanning area, an image signal in high quality can be obtained.
The adjusting unit should preferably adjust the exposure of the photodetector at an absolute point of the effective picture of the optical image so as to fall within its overflow preventive range and adjust the exposure thereof at a brightest point of an effective picture of the optical image so as to approach the quantization exposure limit of the photodetector.
When the original is either a film or a reflective original and the optical image includes a high intensity area that is an image of a luminescent point on the original, the adjusting unit preferably adjusts an exposure of the photodetector in the high intensity area of the optical image to fall within the overflow preventive range of the photodetector, and adjust the exposure of the photodetector at the brightest point of the effective picture of the optical image to approach the quantization exposure limit of the photodetector.
The adjusting unit should preferably adjust the exposure of the photodetector at an absolute brightness point of the effective picture of the optical image so as to fall within its overflow preventive range and adjust the exposure thereof at a brightest point of an effective picture of the optical image so as to approach the quantization exposure limit of the photodetector.
When the optical image includes a high intensity area that is an optical image of an part of the original where the scanning light transmits and/or an area where the original is not arranged, the adjusting unit should preferably adjust the exposure of the photodetector in the high intensity area of the optical image so as to fall within the overflow preventive range of the photodetector, and adjust the exposure of the photodetector at the brightest point of the effective picture of the optical image so as to approach the quantization exposure limit of the photodetector.
To achieve the foregoing objects, the control program of the image scanning apparatus in the present invention is provided. The image scanning apparatus comprises: a radiation unit for radiating a scanning light onto a scanning area where an original is arranged; and a photodetector having an overflow drain mechanism, for detecting an optical image formed by the scanning light which is reflected in the scanning area or transmits therethrough. The control program includes: a test drive procedure for driving the radiation unit and the photodetector, for which a predetermined drive condition is set; and an adjusting procedure for adjusting the drive condition of the radiation unit and the photodetector in accordance with an output signal outputted from the photodetector by the driving. In the adjusting procedure, an exposure of the photodetector at a brightest point of an effective picture of the optical image is adjusted to approach the quantization exposure limit of the photodetector.
In the adjusting procedure, the exposure of the photodetector at an absolute brightness point of the optical image should be preferably adjusted to fall within the overflow preventive range of the photodetector, and the exposure of the photodetector at a brightest point of an effective picture of the optical image should be preferably adjusted to approach the quantization exposure limit of the photodetector.
When the original is either a film or a reflective original and the optical image includes a high intensity area that is an image of a luminescent point on the original, in the adjusting procedure an exposure of the photodetector in the high intensity area of the optical image is preferably adjusted to fall within the overflow preventive range of the photodetector, and the exposure of the photodetector at the brightest point of the effective picture of the optical image is preferably adjusted to approach the quantization exposure limit of the photodetector.
When the original is a negative film having a base and the optical image includes a high intensity area that is an optical image of the base, in the adjusting procedure the exposure of the photodetector in the high intensity area of the optical image is adjusted to fall within the overflow preventive range of the photodetector, and the exposure of the photodetector at the brightest point of the effective picture of the optical image is preferably adjusted to approach the quantization exposure limit of the photodetector.
When the optical image includes a high intensity area that is an optical image of a part of the original where the scanning light transmits and/or an area where the original is not arranged, in the adjusting procedure the exposure of the photodetector in the high intensity area of the optical image is preferably adjusted to fall within the overflow preventive range of the photodetector, and the exposure of the photodetector at the brightest point of the effective picture of the optical image is preferably adjusted to approach the quantization exposure limit of the photodetector.
To achieve the foregoing object, the storage medium for storing the control program of the image scanning apparatus in the present invention is provided. The storage medium comprises: a radiation unit for radiating a scanning light onto a scanning area where an original is arranged; and a photodetector having an overflow drain mechanism, for detecting an optical image formed by the scanning light which is reflected in the scanning area or transmits therethrough. The control program includes: a test drive procedure for driving the radiation unit and the photodetector, for which a predetermined drive condition is set; and an adjusting procedure for adjusting the drive condition of the radiation unit and the photodetector in accordance with an output signal outputted from the photodetector by the driving. In the adjusting procedure, an exposure of the photodetector at a brightest point in an effective picture of the optical image is adjusted to approach the quantization exposure limit of the photodetector.
In the adjusting procedure, the adjustment is preferably performed such that the exposure of the photodetector at an absolute brightness point of the optical image falls within the overflow preventive range of the photodetector, and the exposure of the photodetector at a brightest point of an effective picture of the optical image approaches the quantization exposure limit of the photodetector.
When the original is either a film or a reflective original and a high intensity area that is an image of a luminescent point on the original is included in the optical image, in the adjusting procedure the exposure of the photodetector in the high intensity area of the optical image is preferably adjusted to fall within the overflow preventive range of the photodetector, and the exposure of the photodetector at the brightest point of the effective picture of the optical image is preferably adjusted to approach the quantization exposure limit of the photodetector.
When the original is a negative film having a base and the optical image includes a high intensity area that is an optical image of the base, the adjustment is preferably performed in the adjusting procedure such that the exposure of the photodetector in the high intensity area of the optical image falls within the overflow preventive range of the photodetector, and the exposure of the photodetector at a brightest point of an effective picture of the optical image approaches the quantization exposure limit of the photodetector.
When the optical image includes a high intensity area that is an optical image of a part of the original where the scanning light transmits and/or an area where the original is not arranged, the adjustment is preferably performed in the adjusting procedure such that the exposure of the photodetector in the high intensity area of the optical image falls within the overflow preventive range of the photodetector, and the exposure of the photodetector at the brightest point of the effective picture of the optical image approaches the quantization exposure limit of the photodetector.